A preliminary detailed geologic map, partially supported with funds under STATEMAP, has been completed for Orange County, NC. The majority of the County is underlain by greenschist facies crystalline lithologies that are part of the Hyco Formation of the Virgilina sequence of the Carolina terrane. The Hyco Formation units consists of ca. 615 to 633 Ma layered meta-volcaniclastic rocks and meta-plutonic rocks. Available age dates indicate the Hyco Formation in Orange and Durham Counties may be divided into lower (ca. 630 Ma) and upper (ca. 615 Ma) members (informal) with an apparent intervening hiatus of magmatism. Map units of meta-volcaniclastic rocks include various lithologies that when grouped together are interpreted to indicate general environments of deposition (or lithofacies). The dacitic lavas and tuffs unit is interpreted to represent dacitic domes and proximal pyroclastics. The felsic tuffs units are interpreted to represent air fall tuffs and associated volcanosedimentary rocks deposited distal from a vent. The andesitic to basaltic lavas and tuffs unit is interpreted to represent eruption of intermediate to mafic lava flows and associated pyroclastic deposits. The epiclastic and epiclastic/pyroclastic units are interpreted to represent deposition from the erosion of dormant and active volcanic highlands.

In southern Orange County, Hyco Formation units are intruded by the ca. 579 Ma East Farrington pluton and associated West Farrington pluton. Original layering of Hyco Formation lithologies ranges from shallowly to steeply dipping due to open to isoclinal folds that are locally overturned to the southeast. The northwestern corner of the County is underlain by the Prospect Hill pluton. The Prospect Hill pluton intrudes foliated and folded Hyco Formation units and is correlated with the ca. 546 Ma Roxboro pluton.

The southeastern corner of the County is underlain by Triassic-aged sedimentary rocks of the Durham sub-basin of the Deep River Mesozoic basin. Sills and dikes of Jurassic-aged diabase intrude the Triassic sediments. Diabase dikes also intrude the crystalline rocks of the County. Quaternary aged alluvium is present in most major drainages throughout the County.